1. Field of the Invention
This invention relates to an improved method of manufacturing an electronic device having a multilayer wiring structure wherein the insulation layer or layers formed therein are comprised of a polyimide.
2. Description of the Prior Art
A thermosetting polyimide resin has been heretofore widely used as an insulation layer-forming material for electronic devices having a multilayer wiring structure, such as semiconductor devices, bubble memory devices and thin-film magnetic heads. The application of a thermosetting polyimide resin is described in, for example, Japanese Patent Publication No. 44,871/1976 and Japanese Laid-open Patent Application No. 135,713/1977. The heretofore used polyimide resin is of the type which is cured through a condensation reaction represented, for example, by the following reaction formula: ##STR1## wherein R' is a divalent aromatic radical having no active hydrogen, X' is, for example, ##STR2## and n is a positive integer.
Namely, a polyamic acid represented by the formula (I), or its functional derivative, is condensed, when heated, to be converted to a polyimide having imide rings, represented by the formula (II), while water or other low molecular weight substances are formed. Such a thermosetting polyimide resin of the type formed through the condensation of a polyamic acid or its functional derivatives, is hereinafter referred to as a "condensational type polyimide" for brevity. Although the condensational type polyimide results in an insulation layer having good thermal resistance, it causes some problems, as mentioned below, in the manufacture of the electronic devices, because of the production of water or other low molecular weight substances in the curing step.
For example, semiconductor devices having an insulation layer or layers formed from a condensational type polyimide are generally manufactured by the steps of the following sequence, as illustrated in FIG. 1A through FIG. 1E which schematically represent, in cross-section, the sequential steps of manufacturing a semiconductor device.
(i) A semiconductor substrate 1 is prepared, having built-up circuit elements therein, with predetermined portions of the elements being exposed (non-exposed portions are covered with a protective layer 2, such as silicon dioxide), and a first metal layer of wiring 3 of a predetermined pattern is formed on the circuit elements.
(ii) An uncured polyimide resin is applied by a spin coating method to form a layer 4 of a predetermined thickness, and then, the so formed resin layer 4 is pre-cured by heating, for example, at approximately 220.degree. C. (FIG. 1A).
(iii) A photoresist 5 is applied onto the pre-cured polyimide insulation layer 4, followed by exposure to a light through a mask of a predetermined pattern and the subsequent development thereof (FIG. 1B).
(iv) The pre-cured polyimide insulation layer 4 is etched, and then, the photo resist is removed (FIG. 1C).
(v) The pre-cured polyimide insulation layer 4 is completely cured by heating it at approximately 350.degree..
(vi) A second metal layer 6 of wiring of a predetermined pattern is formed on the cured polyimide insulation layer 4 and the first metal layer 3 of wiring (FIG. 1D).
(vii) The surface of the polyimide insulation layer 4 is roughened by plasma etching.
(viii) An uncured polyimide resin is applied by a spin coating method to form a layer 7 of a predetermined thickness, and then, the so formed resin layer 7 is pre-cured (FIG. 1E).
(ix) Where it is desired to manufacture a multilayer construction having three or more layers, the above-mentioned steps (iii) through (vii) are repeated.
(x) Finally, openings for providing electrical continuity to the electrodes are opened in the uppermost pre-cured polyimide layer 7, and then, the whole device is aged at a temperature of approximately 350.degree. C. for approximately 30 minutes.
The single or each polyimide interlayer insulation layer 4 must be completely cured, as stated in the above-mentioned step (v), prior to the subsequent formation of the metal wiring layer 6 thereon. If the metal wiring layer 6 is formed on a polyimide interlayer insulation layer 4 which has been pre-cured, but not yet completely cured, the single or each polyimide interlayer insulation layer 4 undergoes curing during the above-mentioned step (x) of aging, whereby the water or other low molecular weight substances are produced, and thus, the metal wiring layer 6 is caused to bulge. Furthermore, the completely cured polyimide insulation layer 4 exhibits a poor adhesion to the polyimide insulation layer 7 formed on the insulation layer 4.
In bubble memory devices having a multilayer wiring structure, wherein the interlayer insulation layer is comprised of a thermoset condensational type polyimide, the interlayer insulation layer, or a permalloy layer of a predetermined pattern formed on the interlayer insulation layer, is caused to bulge and corrode due to the water and other low molecular weight substances produced when the polyimide interlayer insulation layer is cured. Furthermore, the thermosetting condensational polyimide exhibits leveling on a bubble memory crystal substrate on which the insulation layer of the polyimide is formed, but the leveling is not satisfactory.
In thin film magnetic heads having a multilayer wiring structure, wherein the insulation layers are comprised of a thermosetting condensational type polyimide, the polyimide insulation layers are caused to bulge when the polyimide insulation layers are cured.